Weight loss clinic
 

There guy goes to a weight loss clinic and says he needs to lose 20 lbs. The receptionist sends him upstairs, where he finds a beautiful naked woman with a sign that says "If you catch me, you can screw me." An hour later, he emerges, sated and 20 lbs. lighter.
A month later, he returns and needs to lose 50 lbs. The receptionist sends him upstairs again, but this time there are two girls with the same sign. A day later, he comes out 50 lbs. lighter.
A year later, he returns and needs to lose 100 lbs. He gets sent upstairs again, where he finds a huge gorilla with a sign that reads "If I catch you, I screw you."